In a radio communication system such as LTE (Long-Term Evolution) or LTE-A (LTE-Advanced) developed by the 3GPP (Third Generation Partnership Project) or WiMAX (Worldwide Interoperability for Microwave Access) developed by the IEEE (Institute of Electrical and Electronics Engineers), a base station and a terminal each include one or a plurality of transmission and reception antennas and can realize high-speed data communication using, for example, a MIMO (multiple input multiple output) technique.
Here, in radio communication systems, it is being examined to support MU-MIMO (multiple user MIMO), in which a plurality of terminals perform spatial multiplexing using the same frequency and time resources. In addition, it is being examined to support a CoMP (cooperative multipoint) communication scheme, in which a plurality of base stations cooperatively perform interference coordination. For example, a radio communication system is being examined that adopts a heterogeneous network deployment (HetNet) realized by a macro base station having wide coverage and an RRH (remote radio head) having coverage narrower than that of the macro base station.
In such a radio communication system, if uplink reference signals transmitted by a plurality of terminals have the same characteristics, interference occurs. Here, for example, a method has been proposed for orthogonalizing demodulation reference signals (DMRSs) transmitted by the plurality of terminals to one another in order to reduce or suppress interference between the demodulation reference signals (NPL 1).